Acentriolar

Acentriolar refers to structures or cellular processes that occur without centrioles. In most animal cells, the centrosome contains a pair of centrioles and serves as a primary microtubule organizing center (MTOC) for organizing the spindle during cell division. An acentriolar context describes spindle formation and MTOC organization that does not depend on centrioles.

Centrioles are absent or not required in several plant and many algal cells, which assemble mitotic spindles using acentriolar MTOCs and chromatin-mediated microtubule nucleation. In many animal oocytes, meiosis proceeds with an acentriolar spindle, where chromosomes and chromatin-associated cues drive spindle assembly rather than a centriole-based centrosome.

Mechanisms involved in acentriolar spindle formation include chromatin-mediated microtubule nucleation, the gamma-tubulin ring complex, and spindle-pole–associated factors such as augmin and TPX2. The Ran-GTP gradient around chromosomes promotes local nucleation and organization of microtubules, enabling spindle assembly in the absence of centrioles. Spindle poles in acentriolar contexts often form from dispersed microtubule organizing foci rather than from centriole-containing centrosomes.

Significance of acentriolar spindle assembly lies in illustrating the flexibility of the cellular machinery to organize division without centrioles. This mode of spindle formation is relevant to developmental biology and to understanding conditions where centriole number or function is altered. See also centriole, centrosome, spindle, and microtubule organizing centers.